1. Field of the Invention: This invention is directed to an ash classifier for use in combination with a fluidized bed reactor.
2. Description: Fluidized bed reactors are well known means for generating heat and, in various forms, can carry out the processes of drying, roasting, calcining, incineration and heat treatment of solids with gases in the chemical, metallurgical and other material processing fields. They are also used for the generation of hot gases, including steam, for use in driving electric power generation equipment, for process heat, for space heating, or for other purposes.
Fluidized bed reactors typically comprise a vessel having a substantially horizontal air distributor or constriction plate, which supports a bed of particulate solids in the reaction chamber and separates the reaction chamber from a windbox below the air distributor. Combustion air is introduced into the windbox and passes through the air distributor in sufficient volume to achieve a gas velocity that expands or fluidizes the solids bed, suspending the particulate solids of the bed in the flowing air stream and imparting to the individual particles a continuous random motion. A fluidized bed in appearance and properties resembles a boiling liquid. Conducting a combustion reaction in a fluidized bed has important advantages which include attainment of a substantially uniform bed temperature, combustion at relatively low temperatures and a high heat transfer rate.
Combustion of solid fuels such as coal involves the gasification of the organic component of the fuel leaving a residue of solid ash particles. The very finest ash particles will be elutriated by the gases flowing in the reactor and will exit through the stack with the exhaust gases. A large portion of the ash particles of a larger particle size will become part of the fluidized bed where they improve the operation of the fluidized bed by retaining heat and contacting and igniting fresh fuel particles. The elevated temperature of the fluidized bed and the continuous motion of the ash particles in that fluidized bed brings about numerous collisions between ash particles in a softened condition due to the elevated temperature. Under these conditions, ash agglomerates readily form and these agglomerates grow to a size such that they are no longer fluidizable and they descend to the bottom of the fluidized bed coming to rest upon the air distribution plate. An accumulation of large ash particles and large ash agglomerates on the air distribution plate will ultimately cause defluidization of the fluidized bed and subsequent shut-down to remove the unfluidized ash accumulation.
This problem has been recognized for some time and various schemes and devices have been put forth to remedy this condition. It is the purpose of the present invention to provide a simple but effective means for removing excess coarse ash particles and oversized ash agglomerates from the fluidized bed while returning the fines removed to the bed to maintain the fluidized condition of the bed.